Coupled Biphase (1T-2H)-MoSe2 on Mold Spore Carbon for Advanced Hydrogen Evolution Reaction

Nitrogen-doped Fe7S8 as highly efficient electrocatalysts for the hydrogen evolution reaction

The high unoccupied d band energy of FeS2 basically results in weak orbital coupling with water molecules, consequently leading to sluggish water dissociation kinetics. Herein, we demonstrate that the N-induced doping effect and phase transition engineering (FeS2 to N-Fe7S8) can downshift the unoccupied d orbitals and strengthen the interfacial orbital coupling to boost the water dissociation kinetics. The fabricated N-Fe7S8/carbon cloth (CC) displays superb hydrogen evolution reaction performance with a low overpotential (89 mV at 10 mA cm-2) and small Tafel slope (105 mV dec-1) under alkaline conditions. It is revealed that the electronic structure of Fe is modulated by N doping and phase transition. The downshifted d band energy can strengthen water adsorption and reduce the energy barrier of water dissociation. Our work provides a new strategy to modify metal sulfide electrocatalysts for electrochemical energy conversion.

Nanohole-Array Induced Metallic Molybdenum Selenide Nanozyme for Photoenhanced Tumor-Specific Therapy

Deficient catalytic sensitivity to the tumor microenvironment is a major obstacle to nanozyme-mediated tumor therapy. Electron transfer is the intrinsic essence for a nanozyme-catalyzed redox reaction. Here, we developed a nanohole-array-induced metallic molybdenum selenide (n-MoSe2) that is enriched with Se vacancies and can serve as an electronic transfer station for cycling electrons between H2O2 decomposition and glutathione (GSH) depletion. In a MoSe2 nanohole array, the metallic phase reaches up to 84.5%, which has been experimentally and theoretically demonstrated to exhibit ultrasensitive H2O2 responses and enhanced peroxidase (POD)-like activities for H2O2 thermodynamic heterolysis. More intriguingly, plenty of delocalized electrons appear due to phase- and vacancy-facilitated band structure reconstruction. Combined with the limited characteristic sizes of nanoholes, the surface plasmon resonance effect can be excited, leading to the broad absorption spectrum spanning of n-MoSe2 from the visible to near-infrared region (NIR) for photothermal conversion. Under NIR laser irradiation, metallic MoSe2 is able to induce out-of-balance redox and metabolism homeostasis in the tumor region, thus significantly improving therapeutic effects. This study that takes advantage of phase and defect engineering offers inspiring insights into the development of high-efficiency photothermal nanozymes.

Fabrication of multiphase MoSe2 modified BiOCl nanosheets for efficient piezo-photoelectric hydrogen evolution and antibiotic degradation

Efficient spatial charge separation plays a crucial role in improving the photocatalytic performance. Therefore, 1T/2H MoSe2/BiOCl (1T/2H MS/BOC) and 2H MoSe2/BiOCl (2H MS/BOC) piezo-photocatalysts are synthesized. By combining piezoelectric catalysis and photocatalysis, a highly active piezo-photocatalytic process is realized. The optimal 1T/2H MS/BOC piezo-photocatalyst displays superior diclofenac (DCF) degradation and hydrogen (H2) evolution activity under the combined action of ultrasound and light. In particular, the DCF degradation kinetic constant (k) of optimal 0.5% 1T/2H MS/BOC under the synergistic effect of ultrasound and light is 0.057 min-1, which is 8.1 and 6.3 times higher than those of BiOCl (0.007 min-1) and 0.5% 2H MS/BOC (0.009 min-1). Moreover, the H2 evolution rate of 0.5% 1T/2H MS/BOC is 122.5 μmol g-1 h-1, which is also higher than those of BiOCl (45.8 μmol g-1 h-1) and 2H MS/BOC (49.5 μmol g-1 h-1). The dramatic improvement in the DCF degradation and H2 evolution piezo-photocatalytic performance of 1T/2H MS/BOC catalysts is ascribed to the built-in polarization electric field and abundance of active sites of 1T/2H MS/BOC as well as the advantageous band structure between BiOCl and 1T/2H MoSe2. Additionally, three probable degradation pathways of DCF were put forward from the results of liquid chromatography-mass spectrometry (LCMS) and density functional theory (DFT) calculations. This study provides the design strategy of high efficiency piezo-photocatalysts in environmental purification and energy-generation fields based on phase and band structure engineering.

Phase-Dependent Dual Discrimination of MoSe2/MoO3 Composites Toward N,N-Dimethylformamide and Triethylamine at Room Temperature

Herein, we present, a chemiresistive-type gas sensor composed of two-dimensional 1T-2H phase MoSe2 and MoO3. Mixed phase MoSe2 and MoSe2/MoO3 composites were synthesized via a facile hydrothermal method. The structure analysis using X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy revealed the formation of different phases of MoSe2 at different temperatures. With increase in synthesis temperature from 180 to 200 °C, the relative percentage of 1T and 2H-MoSe2 phases changed from 80 to 48%. On the other hand, at 220 °C, 2H-MoSe2 was obtained as a major component. The gas sensing properties of individual MoSe2 and composites were investigated at room temperature toward various analytes. The obtained results revealed that composites possess improved sensing features as compared with individual MoSe2 or MoO3. Data also revealed that the composite with dominating 1T-phase exhibits relatively higher response (10%, at 10 ppm) for dimethylformamide (DMF) compared to triethylamine (TEA) (3%, at 10 ppm). In contrast, the composite with larger 2H-phase exhibited affinity toward TEA and had a relative response of about 2%. Therefore, selectivity of a sensor device can be tuned by an appropriately designed MoSe2/MoO3 composite. These results signify the importance of MoO3-based composites with dual-phase MoSe2 for successfully discriminating between DMF and TEA at room-temperature.

MoSe2-NiSe dual co-catalysts modified g-C3N4 for enhanced photocatalytic H2 generation

Solar energy conversion into hydrogen (H₂) energy has attracted much attention. However, the low light utilization rate and fast carrier recombination of photocatalysts extremely limit the practical application of photocatalytic H₂ production. In this paper, MoSe₂-NiSe with abundant active sites and interfacial electronic structures as dual co-catalysts were assembled on g-C₃N₄ nanosheets (NSs) vis a solvothermal reaction process. MoSe₂-NiSe/g-C₃N₄ NSs composite exhibited improved light absorption and photoelectrochemical properties. The photocatalytic H₂ production rate of MoSe₂-NiSe/g-C₃N₄ composite achieved 2379.04 μmol·h^-1·g^-1, which is 99.25, 1.44, and 3.67 times those of pure g-C₃N₄ nanosheets (23.97 μmol·h^-1·g^-1), MoSe₂/C₃N₄ (1654.57 μmol·h^-1·g^-1), and NiSe/C₃N₄ (649.08 μmol·h^-1·g^-1), respectively. The apparent quantum efficiency (AQE) value of MoSe₂-NiSe/g-C₃N₄ achieved 4.07 % under light at λ = 370 nm. The corresponding characterization and experiments proved that 2D ultrathin g-C₃N₄ NSs with a large surface area and short charge-transfer distance could facilitate light scattering and the transport of photoexcited electrons. MoSe₂-NiSe, as a dual co-catalyst, showed strong electronic synergistic interaction between the interfaces, thus improving the conductivity and promoting the electron transfer process.

Activating and optimizing the In-Plane interface of 1 T/2H MoS2 for efficient hydrogen evolution reaction

Implanting the octahedral phase (1 T) into the hexagonal phase (2H) of the molybdenum disulfide (MoS₂) matrix is considered one of the effective methods to enhance hydrogen evolution reaction (HER) performances of MoS₂. In this paper, hybrid 1 T/2H MoS₂ nanosheets array was successfully grown on conductive carbon cloth (1 T/2H MoS₂/CC) via facile hydrothermal method and the 1 T phase content in 1 T/2H MoS₂ is regulated to gradually increase from 0 % to 80 %. 1 T/2H MoS₂/CC with 75 % 1 T phase content exhibits optimal HER performances. The DFT calculation results show that S atoms in 1 T/2H MoS₂ interface exhibit the lowest hydrogen adsorption Gibbs free energies (ΔG_H*) compared with other sites. The improvement of HER performances are primarily attributed to activating the in-plane interface regions of the hybrid 1 T/2H MoS₂ nanosheets. Furthermore, the relationship between 1 T MoS₂ content in 1 T/2H MoS₂ and catalytic activity was simulated by a mathematical model, which shows that the catalytic activity presents a trend of increasing and then decreasing with the increase of 1 T phase content.

Interface prompted highly efficient hydrogen evolution of MoS2/CoS2 heterostructures in a wide pH range

Interfacial electronic characteristics are crucial for the hydrogen evolution reaction (HER), especially in nanoscale heterogeneous catalysts. In this work, we found that the synergistic activation of CoS₂ and MoS₂ (2H-MoS₂ and 1T-MoS₂) greatly enhances the HER activity in a wide pH range compared to those of each component. The Gibbs free energies for hydrogen adsorption at interfacial Co sites are as low as -0.08 (-0.25) eV and -0.20 (0.01) eV for 2H-MoS₂/CoS₂ and 1T-MoS₂/CoS₂ heterostructures in acidic (alkaline) media, respectively, which are even superior to that of Pt(111) (-0.09 eV). Moreover, the theoretical exchange current density of MoS₂/CoS₂ can reach ∼1.98 × 10^-18 A site^-1 (∼8.43 A mg^-1). Experimentally, MoS₂/CoS₂ exhibits a greatly reduced overpotential of 54 (46) mV and a Tafel slope of 42 (50) mV dec^-1 under acidic (alkaline) conditions. The improved performance mainly originates from the synergistically activated interfacial Co atoms with better electron localization and local bonding. The interfacial effect enhances the electron conductivity and improves the H adsorption characteristics, making MoS₂/CoS₂ highly valuable as efficient HER electrocatalysts.

Synergistic effect of intercalation and EDLC electrosorption of 2D/3D interconnected architectures to boost capacitive deionization for water desalination via MoSe2/mesoporous carbon hollow spheres

Transition-metal dichalcogenides can be used for capacitive deionization (CDI) via pseudocapacitive ion intercalation/de-intercalation due to their unique two-dimensional (2D) laminar structure. MoS₂ has been extensively studied in the hybrid capacitive deionization (HCDI), but the desalination performance of MoS₂-based electrodes remains only 20-35 mg g^-1 on average. Benefiting from the higher conductivity and larger layer spacing of MoSe₂ than MoS₂, it is expected that MoSe₂ would exhibit a superior HCDI desalination performance. Herein, for the first time, we explored the use of MoSe₂ in HCDI and synthesized a novel MoSe₂/MCHS composite material by utilizing mesoporous carbon hollow spheres (MCHS) as the growth substrate to inhibit the aggregation and improve the conductivity of MoSe₂. The as-obtained MoSe₂/MCHS presented unique 2D/3D interconnected architectures, allowing for synergistic effects of intercalation pseudocapacitance and electrical double layer capacitance (EDLC). An excellent salt adsorption capacity of 45.25 mg g^- 1 and a high salt removal rate of 7.75 mg g^- 1 min^-1 were achieved in 500 mg L^- 1 NaCl feed solution at an applied voltage of 1.2 V in batch-mode tests. Moreover, the MoSe₂/MCHS electrode exhibited outstanding cycling performance and low energy consumption, making it suitable for practical applications. This work demonstrates the promising application of selenides in CDI and provides new insights for ration design of high-performance composite electrode materials.

MXene Nanosheets Induce Efficient Iron Selenide Active Sites to Boost the Electrocatalytic Hydrogen Evolution Reaction

Along with the widespread utilization of hydrogen energy, the rise of highly active hydrogen evolution electrocatalysts with affordable costs presently becomes a substantial crux of this emerging domain. In this work, we demonstrate a feasible and convenient in situ seed-induced growth strategy for the construction of small-sized FeSe₂ nanoparticles decorated on two-dimensional (2D) superthin Ti₃C₂T_x MXene sheets (FeSe₂/Ti₃C₂T_x) through a manipulated bottom-up synthetic procedure. By virtue of the distinctive 0D/2D heterostructures, abundant exposed surface area, well-distributed FeSe₂ catalytic centers, strong surface electronic coupling, and high electrical conductivity, the resultant FeSe₂/Ti₃C₂T_x nanoarchitectures are endowed with a superior electrocatalytic hydrogen evolution capacity including a competitive onset potential of 89 mV, a favorable Tafel slope of 78 mV dec^-1, and a long-period stability, significantly better than that of the pristine FeSe₂ and Ti₃C₂T_x catalysts.

A Review on Production and Surface Modifications of Biochar Materials via Biomass Pyrolysis Process for Supercapacitor Applications

Biochar (BC) based materials are solid carbon enriched materials produced via different thermochemical techniques such as pyrolysis. However, the non-modified/non-activated BC-based materials obtained from the low-temperature pyrolysis of biomass cannot perform well in energy storage applications due to the mismatched physicochemical and electrical properties such as low surface area, poor pore features, and low density and conductivity. Therefore, to improve the surface features and structure of the BC and surface functionalities, surface modifications and activations are introduced to improve its properties to achieve enhanced electrochemical performance. The surface modifications use various activation methods to modify the surface properties of BC to achieve enhanced performance for supercapacitors in energy storage applications. This article provides a detailed review of surface modification methods and the application of modified BC to be used for the synthesis of electrodes for supercapacitors. The effect of those activation methods on physicochemical and electrical properties is critically presented. Finally, the research gap and future prospects are also elucidated.

Ultrafine cobalt selenide nanowires tangled with MXene nanosheets as highly efficient electrocatalysts toward the hydrogen evolution reaction

Hydrogen energy has attracted sustainable attention in the exploitation and application of advanced power-generator devices, and electrocatalysts for the hydrogen evolution reaction (HER) have been regarded as one of the core components in the current electrochemical hydrogen production systems. In this work, a facile and cost-effective bottom-up strategy is developed for the construction of 1D ultrafine cobalt selenide nanowires tangled with 2D Ti₃C₂T_x MXene nanosheets (CoSe NW/Ti₃C₂T_x) through an in situ stereo-assembly process. Such an architectural design endows the hybrid system not only with a large accessible surface for the rapid transportation of reactants, but also with numerous exposed CoSe edge sites, thereby generating substantial synergic coupling effects. The as-derived CoSe NW/Ti₃C₂T_x hybrid demonstrates competitive electrocatalytic properties toward the HER with a small onset potential of 84 mV, a low Tafel slope of 56 mV dec^-1 and exceptional cycling performance, which are superior to those of bare CoSe and Ti₃C₂T_x materials. It is believed this promising nanoarchitecture may provide new possibilities for the design and construction of precious-metal-free electrocatalysts with high efficiency and great stability in the energy-conversion field.

Coupling of Ru nanoclusters decorated mixed-phase (1T and 2H) MoSe2 on biomass-derived carbon substrate for advanced hydrogen evolution reaction

The development of efficient catalysts for hydrogen evolution reaction (HER) from water splitting is one of the most promising strategies to achieve the goal of peak carbon dioxide emissions and carbon neutrality. Herein, Ru nanoclusters decorated MoSe₂ nanosheets supported on a Crepis tectorum fluff biomass-derived hollow carbon tube (Ru-MoSe₂/CMT) are prepared as the HER catalysts in both alkaline and acidic conditions. The Ru modification induces the transformation of MoSe₂ from 2H phase to 1T phase. Benefiting from the strong water dissociation ability of Ru, Ru-MoSe₂/CMT exhibits a low overpotential of 70 mV with a Tafel slope of 39 mV dec^-1 in 1 M KOH. Furthermore, the assembled Ru-MoSe₂/CMT || RuO₂ system with a low cell voltage of 1.54 V at 10 mA cm^-2 exhibits outstanding overall water splitting performance superior to Pt/C || RuO₂ system. The Ru-MoSe₂/CMT || RuO₂ system also achieves the excellent stability of up to 30 h in 1 M KOH. The synergy effect between Ru and MoSe₂, as well as the improved electron transfer kinetics provided by the biomass-derived carbon substrate together contribute to the excellent HER activity of Ru-MoSe₂/CMT.

Anderson-Type Polyoxometalate-Assisted Synthesis of Defect-Rich Doped 1T/2H-MoSe2 Nanosheets for Efficient Seawater Splitting and Mg/Seawater Batteries

Designing high-performance hydrogen evolution reaction (HER) catalysts is crucial for seawater splitting. Herein, we demonstrate a facile Anderson-type polyoxometalate-assisted synthesis route to prepare defect-rich doped 1T/2H-MoSe₂ nanosheets. As demonstrated, the optimized defect-rich doped 1T/2H-MoSe₂ nanosheets display low overpotentials of 116 and 274 mV to gain 10 mA cm^-2 in acidic and simulated seawater for the HER, respectively. A magnesium (Mg)/seawater battery was fabricated with the defect-rich doped 1T/2H-MoSe₂ nanosheet cathode, displaying the highest power density of up to 7.69 mW cm^-2 and stable galvanostatic discharging over 24 h. The theoretical and experimental investigations show that the superior HER and battery performances of the heteroatom-doped MoSe₂ nanosheets are attributed to both the improved intrinsic catalytic activity (effective activation of water and favorable subsequent hydrogen desorption) and the abundant active sites, benefiting from the favorable catalytic factors of the doped heteroatom, 1T phase, and defects. Our work presents an intriguing structural modulation strategy to design high-performance catalysts toward both HER and Mg/seawater batteries.

A new MoCN monolayer containing stable cyano structural units as a high-efficiency catalyst for the hydrogen evolution reaction

In the hydrogen evolution reaction (HER), it is essential to find a high-efficiency and nonprecious electrocatalyst comparable to Pt, which needs to have rich inherently active sites and good conductivity. By combining a global minimum structure search and first-principles calculations, a hitherto unknown 2D MoCN monolayer was found, which can be considered as a structure in which Mo atoms interact with the stable CN units through triple bonds. The resultant MoCN monolayer possesses superior thermodynamic, dynamic, thermal, and mechanical stabilities, as well as inherent metallicity. In particular, it can exhibit outstanding HER catalytic activity due to the presence of many active sites with near-zero ΔG_H* values, whose density totals 1.80 × 10¹⁵ sites per cm², even more than Pt. In addition, we also propose a series of other 2D monolayers containing stable CN units (i.e., MoC₂N, MoCN₂ and MoC₂N₂), all of which can uniformly show high stability and good HER catalytic activity. Applying strain can further effectively improve the activities of C-rich (MoC₂N) and N-rich (MoCN₂) monolayers, inducing considerably high HER catalytic performance. For the MoCN, MoC₂N and MoCN₂ monolayers, the most active sites are located at the Mo-C-N chain involved. All these fascinating findings can not only provide new excellent candidates but also new insights into the design of highly efficient and nonprecious HER electrocatalysts as an alternative to Pt in the near future.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔG_H* value. Its active site density counts up to 1.46 × 10¹⁵ site/cm², larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H₂ spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.

Interface engineering of hierarchical P-doped NiSe/2H-MoSe2 nanorod arrays for efficient hydrogen evolution

Developing non-noble metal-based electrocatalysts with better activity and stability for hydrogen evolution reaction (HER) is crucial for the electrolysis of water. Herein, self-supported three-dimensional (3D) P-doped NiSe/2H-MoSe2 nanorod arrays (denoted...

Atomically Dispersed Mo Sites Anchored on Multichannel Carbon Nanofibers toward Superior Electrocatalytic Hydrogen Evolution

Developing affordable and efficient electrocatalysts as precious metal alternatives toward the hydrogen evolution reaction (HER) is crucially essential for the substantial progress of sustainable H₂ energy-related technologies. The dual manipulation of coordination chemistry and geometric configuration for single-atom catalysts (SACs) has emerged as a powerful strategy to surmount the thermodynamic and kinetic dilemmas for high-efficiency electrocatalysis. We herein rationally designed N-doped multichannel carbon nanofibers supporting atomically dispersed Mo sites coordinated with C, N, and O triple components (labeled as Mo@NMCNFs hereafter) as a superior HER electrocatalyst. Systematic characterizations revealed that the local coordination microenvironment of Mo is determined to be a Mo-O₁N₁C₂ moiety, which was theoretically probed to be the energetically favorable configuration for H intermediate adsorption by density functional theory calculations. Structurally, the multichannel porous carbon nanofibers with open ends could effectively enlarge the exposure of active sites, facilitate mass diffusion/charge transfer, and accelerate H₂ release, leading to promoted reaction kinetics. Consequently, the optimized Mo@NMCNFs exhibited superior Pt-like HER performance in 0.5 M H₂SO₄ electrolyte with an overpotential of 66 mV at 10 mA cm^-2, a Tafel slope of 48.9 mV dec^-1, and excellent stability, outperforming a vast majority of the previously reported nonprecious HER electrocatalysts. The concept of both geometric and electronic engineering of SACs in this work may provide guidance for the design of high-efficiency molecule-like heterogeneous catalysts for a myriad of energy technologies.

Interfacial Mo-N-C Bond Endowed Hydrogen Evolution Reaction on MoSe2@N-Doped Carbon Hollow Nanoflowers

Molybdenum diselenide (MoSe₂) has been considered as promising electrocatalysts for catalyzing the hydrogen evolution reaction (HER) due to its narrow band gap and appropriate adsorption free energy. However, its catalytic performance is still impeded by inferior electrical conductivity and insufficient active sites, thus leading to unsatisfactory HER performance. Herein, MoSe₂@N-doped carbon (NC) hollow nanoflowers with interfacial Mo-N-C bonds were controllably fabricated through the in situ selenization of the self-polymerized Mo-polydopamine precursor. Benefiting from the unique hollow structure, NC protective layer, and intimate interfacial interaction, the optimal MoSe₂@NC displays good HER performance with low overpotentials (175 and 183 mV) and long-term stability (up to 12 h at -10 mA cm^-2) in 0.5 M H₂SO₄ and 1.0 M KOH solutions, respectively. The theoretical results show that Mo-N-C bonds at the interface of MoSe₂@NC give rise to relatively low unoccupied e_g orbital density of states and ideal H₂ adsorption free energy. This work presented here highlights the critical role of interfacial chemical bonds in regulating the electronic structure of nanomaterials and further improving the HER performance.

Metallic Transition Metal Dichalcogenides of Group VIB: Preparation, Stabilization, and Energy Applications

Layered transition metal dichalcogenides (TMDs) of group VIB have been widely used in the realms of energy storage and conversions. Along with the existence of semiconducting states, their metallic phases have recently attracted numerous attentions owing to their fascinating physical and chemical properties. Many efforts have been devoted to obtain metallic TMDs with high purity and yield. Nevertheless, such metallic phase is thermodynamically metastable and tends to convert into semiconducting phase, which necessitates the exploration over effective strategies to ensure the stability. In this review, typical fabrication routes are introduced and those critical factors during preparation are elaborately discussed. Moreover, the stabilized strategies are summarized with concrete examples highlighting the key mechanisms toward efficient stabilization. Finally, emerging energy applications are overviewed. This review presents comprehensive research status of metallic group VIB TMDs, aiming to facilitate further scientific investigations and promote future practical applications in the fields of energy storage and conversion.

A stable and ultrafast K ion storage anode based on phase-engineered MoSe2

Potassium-ion batteries (PIBs) are attracting increasing attention due to the abundance of K resources, but the sluggish kinetics and inferior cycling stability of anodes still hinder their application. Herein, we present a hybrid 1T/2H phase MoSe₂ anode, which shows noticeable pseudocapacitive response and fast kinetics for K storage. Correspondingly, superior electrochemical performances including a high reversible capacity of 440 mA h g^-1 after 100 cycles at 0.1 A g^-1 and superb rate capacity of 211 mA h g^-1 at 20.0 A g^-1 are achieved. We believe this work may shed light on the phase engineering of transition metal compounds for rapid charging PIBs.

Ammonium Intercalation Induced Expanded 1T-Rich Molybdenum Diselenides for Improved Lithium Ion Storage

Transition metal dichalcogenides (TMDs), particularly molybdenum diselenides (MoSe₂), have the merits of their unique two-dimensional (2D) layered structures, large interlayer spacing (∼0.64 nm), good electrical conductivities, and high theoretical capacities when applied in lithium-ion batteries (LIBs) as anode materials. However, MoSe₂ remains suffering from inferior stability as well as unsatisfactory rate capability because of the unavoidable volume expansion and sluggish charge transport during lithiation-delithiation cycles. Herein, we develop a simultaneous reduction-intercalation strategy to synthesize expanded MoSe₂ (e-MoSe₂) with an interlayer spacing of 0.98 nm and a rich 1T phase (53.7%) by rationally selecting the safe precursors of ethylenediamine (NH₂C₂H₄NH₂), selenium dioxide (SeO₂), and sodium molybdate (Na₂MoO₄). It is noteworthy that NH₂C₂H₄NH₂ can effectively reduce SeO₂ and MoO₄^2- forming MoSe₂ nanosheets; in the meantime, the generated ammonium (NH₄⁺) efficiently intercalates between MoSe₂ layers, leading to charge transfer, thus stabilizing 1T phases. The obtained e-MoSe₂ exhibits high capacities of 778.99 and 611.40 mAh g^-1 at 0.2 and 1 C, respectively, together with excellent cycling stability (retaining >90% initial capacity at 0.2 C over 100 charge-discharge cycles). It is believed that the material design strategy proposed in this paper provides a favorable reference for the synthesis of other transition metal selenides with improved electrochemical performance for battery applications.

Promoting effect of MXenes on 1T/2H–MoSe2 for hydrogen evolution

The 1T/2H–MoSe₂/Ti₃C₂ composites integrated via a facile hydrothermal method exhibit an optimal overpotential of 150 mV at 10 mA cm⁻² in 1 M KOH, indicating that Ti₃C₂ is an ideal conductive support for building highly efficient electrocatalysts.

Transforming Photocatalytic g-C3 N4 /MoSe2 into a Direct Z-Scheme System via Boron-Doping: A Hybrid DFT Study

Z-scheme photocatalytic systems are an ideal band alignment structure for photocatalysis because of the high separation efficiency of photo-induced carriers while simultaneously preserving the strong reduction activity of electrons and oxidation activity of holes. However, the design and construction of Z-scheme photocatalysts is challenging because of the need for appropriate energy band alignment and built-in electric field. Here, we propose a novel approach to a Z-scheme photocatalytic system using density functional theory calculations with the HSE06 hybrid functional. The undesirable type-I g-C₃ N₄ /MoSe₂ heterojunction is transformed into a direct Z-scheme system through boron doping of g-C₃ N₄ (B-doped C₃ N₄ /MoSe₂ ). Detailed analysis of the total and partial density of states, work functions and differential charge density distribution of the B-doped C₃ N₄ /MoSe₂ heterojunction shows the proper band alignment and existence of a built-in electric field at the interface, with the direction from g-C₃ N₄ to MoSe₂ , demonstrating a direct Z-scheme heterojunction. Further investigation on the absorption spectra reveals a large enhancement of the light absorption efficiency after boron doping. The results consistently confirm that electronic structures and photocatalytic performance can be effectively manipulated by a facile boron doping. Modulating the band alignment of heterojunctions in this way provides valuable insights for the rational design of highly efficient heterojunction-based photocatalytic systems.

Coupling PEDOT on Mesoporous Vanadium Nitride Arrays for Advanced Flexible All-Solid-State Supercapacitors

Tailored construction of advanced flexible supercapacitors (SCs) is of great importance to the development of high-performance wearable modern electronics. Herein, a facile combined wet chemical method to fabricate novel mesoporous vanadium nitride (VN) composite arrays coupled with poly(3,4-ethylenedioxythiophene) (PEDOT) as flexible electrodes for all-solid-state SCs is reported. The mesoporous VN nanosheets arrays prepared by the hydrothermal-nitridation method are composed of cross-linked nanoparticles of 10-50 nm. To enhance electrochemical stability, the VN is further coupled with electrodeposited PEDOT shell to form high-quality VN/PEDOT flexible arrays. Benefiting from high intrinsic reactivity and enhanced structural stability, the designed VN/PEDOT flexible arrays exhibit a high specific capacitance of 226.2 F g^-1 at 1 A g^-1 and an excellent cycle stability with 91.5% capacity retention after 5000 cycles at 10 A g^-1 . In addition, high energy/power density (48.36 Wh kg^-1 at 2 A g^-1 and 4 kW kg^-1 at 5 A g^-1 ) and notable cycling life (91.6% retention over 10 000 cycles) are also achieved in the assembled asymmetric flexible supercapacitor cell with commercial nickel-cobalt-aluminum ternary oxides cathode and VN/PEDOT anode. This research opens up a way for construction of advanced hybrid organic-inorganic electrodes for flexible energy storage.

Recent progress in transition metal selenide electrocatalysts for water splitting

The urgent demand of scalable hydrogen production has motivated substantial research on low cost, efficient and robust catalysts for water electrolysis. In order to replace noble metals and their derivatives, transition metal (Fe, Co, Ni, Mo, Cu, etc.) selenides have demonstrated promising catalysis on both hydrogen and oxygen evolutions. Very recently, a number of reports have presented a variety of approaches to enhance their electrocatalytic activity. This review summarizes the most recent progress in transition metal selenide electrocatalysts for HER, OER, and overall water splitting. The merits and limitations of metal selenides are also discussed in the aspects of structure and composition. Moreover, we highlight new strategies and future challenges for design and synthesis of high performance electrocatalysts.

Se-Rich MoSe2 Nanosheets and Their Superior Electrocatalytic Performance for Hydrogen Evolution Reaction

Two-dimensional MoSe₂ has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), although its catalytic activity needs to be further improved. Herein, we report Se-rich MoSe₂ nanosheets synthesized using a hydrothermal reaction, displaying much enhanced HER performance at the Se/Mo ratio of 2.3. The transition from the 2H to the 1T' phase occurred as Se/Mo exceeded 2. Structural analysis revealed the presence of Se adatoms as well as the formation of Se-Se bonding. Based on first-principles calculations, we propose two equally stable Se-rich structures. In the first one, excess Se atoms bridge two MoSe₂ layers via the interlayer Se-Se bonds. In the second one, the Se atoms substitute for the Mo atoms, and extra Se atoms are added closest to the Mo-substituted Se. Calculation of Gibbs free energy along the reaction path indicates that the Se adatoms of the second model are the most active sites for HER.

Construction of 1T-MoSe2 /TiC@C Branch-Core Arrays as Advanced Anodes for Enhanced Sodium Ion Storage

The use of active sites and reaction kinetics of MoSe₂ anodes for sodium ion batteries (SIBs) are highly related to the phase components (1T and 2H phases) and electrode architecture. This study concerns the design and fabrication of wrinkled 1T-MoSe₂ nanoflakes anchored on highly conductive TiC@C nanorods to form 1T-MoSe₂ /TiC@C branch-core arrays by a powerful chemical vapor deposition (CVD)-solvothermal method. The 1T-MoSe₂ branch can be easily transformed into its 2H-MoSe₂ counterpart after a facile annealing process. In comparison to 2H-MoSe₂ , 1T-MoSe₂ has larger interlayer spacing and higher electronic conductivity, which are beneficial for the acceleration of reaction kinetics and capacity improvement. In addition, direct growth of 1T-MoSe₂ nanoflakes on the TiC@C skeleton not only enhance the electrical conductivity, but also contribute to reinforced structural stability. Accordingly, 1T-MoSe₂ /TiC@C branch-core arrays are demonstrated with higher capacity and better rate performance (184 mAh g^-1 at 10 A g^-1 ) and impressive durability over 500 cycles with a capacity retention of approximately 91.8 %. This phase modulation plus branch-core design provides a general method for the synthesis of other high-performance electrode materials for application in electrochemical energy storage.

Controllable Synthesis of Two-Dimensional Molybdenum Disulfide (MoS2 ) for Energy-Storage Applications

Lamellar molybdenum disulfide (MoS₂ ) has attracted a wide range of research interests in recent years because of its two-dimensional layered structure, ultrathin thickness, large interlayer distance, adjustable band gap, and capability to form different crystal structures. These special characteristics and high anisotropy have made MoS₂ widely applicable in energy storage and harvesting. In this Minireview, a systematic and comprehensive introduction to MoS₂ , as well as its composites, is presented. It is aimed to summarize the various synthetic methods of MoS₂ -based composites and their application in energy-storage devices (lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, and supercapacitors) in detail. Based on recent studies, this Minireview provides important and comprehensive guidelines for further study and development efforts in the MoS₂ in energy-storage field.

An integrated electrode based on nanoflakes of MoS2 on carbon cloth for enhanced lithium storage

Due to its high specific capacity (in theory), molybdenum disulfide (MoS₂) has been recognized as a plausible substitute in lithium-ion batteries (LIBs). However, it suffers from an inferior electric conductivity and a substantial volume change during Li⁺ insertion/extraction. By using a facile hydrothermal method, a flexible free-standing MoS₂ electrode has here been fabricated onto a carbon cloth substrate. The grafting of ultrathin MoS₂ nanoflakes onto the carbon cloth framework (forming CC@MoS₂), was shown to facilitate an improved electron transport, as well as an enhanced Li⁺ transport. As expected, the as-obtained CC@MoS₂ electrode was observed to exhibit an excellent lithium storage performance. It delivers a high discharge specific capacity of 2.42 mA h cm^-2 at 0.7 mA cm^-2 (even after 100 cycles), which is an impressive result.

Understanding the Role of Nanoscale Heterointerfaces in Core/Shell Structures for Water Splitting: Covalent Bonding Interaction Boosts the Activity of Binary Transition-Metal Sulfides

The appropriate catalyst model with a precisely designed interface is highly desirable for revealing the real active site at the atomic level. Herein, we report a proof-of-concept strategy for creating an exposed and embedding interface model by constructing a unique Co₉S₈ core with a full WS₂ shell (Co₉S₈/FWS₂) and a half WS₂ shell (Co₉S₈/HWS₂) to uncover the synergistic effect of heterointerfaces on the catalytic performances. Tailoring the heteroepitaxial growth of WS₂ shell, Co₉S₈/HWS₂ with exposed Co-S-W interfaces leads to the exceptional electron density changes on edged-S atoms with large amounts of lone-pair electrons. Meanwhile, the unique Co₉S₈/HWS₂ could accelerate the kinetic adsorption of hydrogen- and oxygen-containing intermediates. Such Co₉S₈/HWS₂ electrocatalysts show extremely low overpotentials of 78 and 290 mV at a current density of 10 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction, respectively. Using Co₉S₈/HWS₂ as both the cathode and anode, an alkali electrolyzer delivers a current density of 10 mA cm^-2 at a quite low cell voltage of 1.60 V. The results of both operando Raman spectroscopy and electron spin resonance indicate the presence of S-S terminal and S-S bridging with unsaturated S atoms during the HER process. The present work reveals the synergistic effects of nanoscale interfaces on overall electrocatalytic water splitting.

Construction of hierarchical yolk-shell nanospheres organized by ultrafine Janus subunits for efficient overall water splitting

Although the bimetal sulfide is intensively pursued in catalytic systems, synthesis of ultrafine sized bimetal sulfide Janus subunits is still a great challenge. In this work, ultrafine NiS₂/MoS₂ Janus subunits organized on yolk-shell nanospheres (NSs) are synthesized by a novel and facile approach. The greatly reduced particle size of both two-dimensional MoS₂ and one-dimensional NiS₂ on the ultrafine NiS₂/MoS₂ Janus subunits endows the yolk-shell NSs with numerous intimate interfaces of bimetal sulfide hybrids greatly promoting the intimate electronic interaction and dissociation of water molecules. Benefiting from the ultrafine NiS₂/MoS₂ Janus subunits, abundant edge sites and the high density of interfaces, the as-prepared NiS₂/MoS₂ yolk-shell NSs exhibit high electrocatalytic activity with a low η₁₀ value of 135 and 293 mV for the HER and OER, respectively. In addition, a low cell voltage (1.58 V) is achieved by using NiS₂/MoS₂ yolk-shell NSs as both anode and cathode. This study has significant indications in exploring the ultrafine nanoparticles for the water splitting reaction, fuel cells and organic synthesis.

Synthesis of an iron-doped 3D-ordered mesoporous cobalt phosphide material toward efficient electrocatalytic overall water splitting

The development of porous metal phosphides with abundant active sites is of great importance for efficient electrocatalytic water splitting.

Continuous phase regulation of MoSe2 from 2H to 1T for the optimization of peroxidase-like catalysis

Simultaneous and synergistic modulation of the crystal phase and disorder in MoSe₂ to dramatically enhance their peroxidase-like activity.

Unveiling the Origin of the High Catalytic Activity of Ultrathin 1T/2H MoSe2 Nanosheets for the Hydrogen Evolution Reaction: A Combined Experimental and Theoretical Study

2 D transition metal dichalcogenide materials with layered nanostructures and specific phases usually exhibit excellent catalytic activities for the hydrogen evolution reaction (HER). A facile solvothermal process was used to prepare ultrathin noble-metal-free 2 D biphasic MoSe₂ nanosheets composed of a metastable metallic 1T phase and a semiconducting 2H phase. High metallic 1T phase content and few-layer-thick MoSe₂ nanosheets were obtained by tuning the amount of NaBH₄ used in the reaction. The optimal integration of a metallic 1T phase and an environmentally stable 2H phase in MoSe₂ electrocatalysts provides abundant active sites and good conductivity beneficial for the HER. The combination of experimental results and DFT calculations implies that the electrocatalytic activity for the HER on the MoSe₂ surface goes through a collaborative Heyrovsky and Volmer reaction process. The theoretical studies suggest that the presence of 1T-MoSe₂ could reduce the band energy relative to 2H-MoSe₂ and, consequently, accelerate the sluggish HER kinetics of 2H-MoSe₂ . This work provides valuable and novel insights into the understanding of the structure-activity relationships in 2 D transition metal dichalcogenide electrocatalysts.